Experimental: unify sidebar row geometry

[brsbl]

Human comments

What was wrong

Sidebar row families independently owned status, caret, identity, action, and nesting geometry, making column alignment and spacing changes drift across row types.

What changed

• Introduce the experimental shared SidebarRow rail and column system.
• Migrate thread, project, worktree, section, lifecycle, and top-level rows to the shared geometry.
• Centralize row spacing, depth, status, caret, action, and responsive hit-target behavior.
• Add focused contract coverage for the shared row system and migrated row families.

No public or wire contracts changed.

Screenshots

Pending; this PR is intentionally experimental and not merge-ready.

How you verified

• git diff --check
• Remote CI currently fails; this draft is not merge-ready.
• Exact branch web-app verification pending.
• Per repository policy, no CI-equivalent tests, typechecks, lint, or builds were run locally.

Fixes #

BB-Thread-ID: thr_ccffp4w2p2

AGENT GENERATED

[brsbl]

Closing this experimental layer for now. The approved sidebar and command-palette stack remains separate; any future thread-list work will restart incrementally from the stable baseline. Branch retained for reference.